Optical media such as compact discs (CDs) and digital versatile discs (DVDs) store data that is read back optically. CDs typically include a substrate that is made of plastic. For pre-recorded CDs, the substrate may be impressed during manufacturing with microscopic bumps that are arranged as a continuous spiral track of data. The bumps correspond to pits (from the opposite side) and lands are located between adjacent pits. A layer such as aluminum may be formed on the substrate to cover the bumps. A protective layer such as an acrylic may also be applied over the aluminum layer.
A DVD typically includes several layers. For pre-recorded DVDs, each layer may be impressed during manufacturing with microscopic bumps that are arranged as a spiral track of data. A reflective layer such as aluminum may be applied to the substrate to cover the bumps. A semi-reflective layer such as gold may be used for the outer layers. The semi-reflective layer allows the laser pass through onto the inner layers. The layers are coated with lacquer, aligned, laminated and cured.
During read back, the CD and/or DVD is rotated by the optical media playback device, which typically includes (amongst other things) a laser, a spindle motor and an optical sensor. The spindle motor rotates the optical medium. The laser is directed onto the tracks of the optical medium and the optical sensor measures reflected light. When the optical sensor generates a high current level corresponding to high reflectivity, the data may be interpreted as a “1” (or “0”). When the optical sensor generates a low current level corresponding to low reflectivity, the data may be interpreted as a “0” (or “1”). In some devices, the land/pit signal, or commonly described as converted non-return to zero inverted (NRZI) signal may be converted to a non-return to zero (NRZ) signal, as shown in FIG. 1, where 1's represent transitions and 0's represent the absence of transitions.
The data that is stored on the CD and/or DVD typically includes alternating sync patterns and data blocks. The sync patterns are used to identify the data blocks. In other words, the optical media playback device must be able to identify the location of the sync patterns so that the corresponding data block can be read properly. The optical media playback device also typically includes a sync detector module that is used to identify the sync patterns.
Defect errors may occur due to scratches, dirt and/or other materials on the optical media. Defect errors may alter the reflectivity of the laser light, which adversely impacts the NRZI converted signal that is generated by the optical sensor. As a result, the NRZI converted signal may include errors. In some situations, the defect errors may cause the sync detector circuit to fail to detect a sync pattern, which may cause the corresponding data block to be lost.
The optical media playback device also includes a timing circuit that generates timing signals for the sync detector circuit. When timing errors occur, transitions in the NRZI converted signal for a sync pattern and/or data will shift left or right from time to time. This kind of shifting pattern will make it more difficult to identify a proper sync pattern. In some circumstances, a faulty sync pattern may be created that looks very similar to a sync pattern carrying defect errors.